Opto-electrical position measuring devices may be used for measuring the relative position of two movable objects. The objects which are movable with relation to each other may be two machine parts of a machine tool, for example, which require the positioning of one tool with relation to a workpiece to be processed. For this purpose a scale is connected with one machine part, the gradation of which is scanned by a scanning unit disposed on the other machine part.
The scale has a base body preferably formed of glass. The graduation is applied to the base body in one of two ways. If the scale is employed with linear or angular measuring systems operating by transmitted light, the graduation is applied to the base body in the form of alternating transparent and opaque areas. If the scale is employed with linear or angular measuring systems operating by incident light, the graduation is applied to the base body in the form of alternating reflecting and non-reflecting areas. With either the transmitted or incident light operation, the graduation may be embodied as an amplitude or phase grating.
With amplitude and phase gratings utilizing incident light, a highly-reflective continuous layer is applied to the base body. It has been shown that mechanical stresses appear in this layer and because of these stresses, the base body becomes deformed. With linear or angular measurements it is required that during scanning, a constant distance between the scale and the scanning unit be maintained over the entire length of the scale. If the scale curves because of stresses caused by the reflecting layer, the scanning distance changes along the length of the scale which causes measuring errors.
European Patent Publication EP 0 112 509 A2 discloses a radiation mask for X-ray lithography intended to prevent deformation by means of a symmetrical layer structure.